1. Field of the Invention
The present invention relates generally to automatic transmissions for use in automotive vehicles and the like, and more specifically to a load responsive engagement pressure control arrangement which reduces shift shock.
2. Description of the Prior Art
JP-A-61-165054 discloses a transmission arrangement which includes a high clutch and a band brake which are selectively engaged and disengaged respectively when a 2-3 shift valve assumes an upshift position and supplies hydraulic fluid thereto. An accumulator is connected with the hydraulic fluid chambers of the high clutch and the band brake by way of a one-way orifice arrangement; During upshifting, the one-way orifice arrangement permits the working chamber of the accumulator to charge freely in an effort to reduce shift shock and then restricts the draining of the working chamber during downshifting.
However, this arrangement has encountered the problem that when an upshift occurs with the accelerator pedal of the vehicle in a non-depressed state (viz., fully released condition), shift shock is produced.
The reason for this is that the accumulator has two back-pressure chambers, one of which is supplied with line pressure from a manual valve during all forward speeds.
In order to ensure that the required amount of torque transmitting engagement of the high clutch is produced during wide open throttle upshifts, the application of the line pressure level back-pressure on a given differential area is necessary in order to restrict the stroking of the accumulator piston. However, during upshifts wherein the accelerator pedal is fully released, the application of the line pressure level back-pressure induces the problem that the stroking of the accumulator piston is overly restricted and the amount of friction element engagement is more than required under the instant set of operating conditions.
On the other hand, if the accumulator is set so that the force produced by the back-pressure under low load is sufficiently low to avoid shift shock with the accelerator pedal fully released, the amount of engagement under high load (e.g. full open throttle) becomes inadequate. This of course leads to slippage and rapid wearing of the elements in question, and/or insufficiently rapid brake release.